1. Field of the Invention
This invention relates generally to magnetic field sensors and more particularly to auto-calibrated magnetic field sensors used as current sensors.
2. Background Art
Magnetic field sensors of all kinds are used in many applications. One type of magnetic field sensor is the current sensor, which is widely used in circuit breaker and metering applications. For instance, many circuit breakers use at least one current transformer to sense the current level in the electrical distribution circuit being protected. The current transformer includes an annular core that encircles the line conductor of the distribution circuit and has a multi-turn winding wound thereon. The current flowing through the line conductor generates a magnetic field that produces flux in the transformer""s core, resulting in an output from the multi-turn winding that is indicative of the current level. If the current exceeds a predetermined level, then the circuit breaker mechanism is tripped. The sensor must be calibrated properly so that the output from the multi-turn winding accurately represents the current in the line conductor.
All such magnetic field sensors are affected by temperature and other environmental factors that can adversely impact the performance of the sensor. Over time, these adverse effects that will affect the calibration of the sensor. The loss of calibration of the sensor will reduce its accuracy.
Accordingly, it would be desirable to have a magnetic field sensor that can calibrate itself and remove the affects of temperature and other environmental factors on its sensitivity.
The above-mentioned need is met by the present invention which provides a magnetic field sensor having a means for producing an output signal that is a function of a magnetic field to be sensed. A calibration coil is arranged so as to change the magnetic field when energized, and a means for selectively energizing the calibration coil is included. Calibration of the magnetic field sensor is accomplished by causing a known current to flow through the calibration coil, measuring the resultant change in the magnetic field, and calculating a sensitivity for the magnetic field sensor from the measured change in the magnetic field and the known current.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.